Method of producing a phthalocyanine photoconductive layer

ABSTRACT

A NOVEL PROCESS FOR THE PREPARATION OF A PHOTOCONDUCTIVE LAYER OF AN ELECTROPHOTOGRAPHIC PLATE WHICH COMPRISES COMBINING PHTAHLOCYANNINE PIGMENT PARTICLES AND A BINDER MATERIAL IN A LIQUID MEDIUM AND SANDMILLING AND THE COMBINATION IS DISCLOSED.

United States Patent Ofice 3,672,979 Patented June 27, 1972 US. Cl.117-401 Claims ABSTRACT OF THE DISCLOSURE A novel process for thepreparation of a photoconductive layer of an electrophotographic platewhich comprises combining phthalocyanine pigment particles and a bindermaterial in a liquid medium and sandmilling and the combination isdisclosed.

BACKGROUND OF THE INVENTION This invention relates to electrophotographyand more particularly to the preparation of a binder plate usable inxerography.

In the art of xerography as originally disclosed by Carlson in US. Pat.2,297,691, an electrostatic latent image is formed on a photoconductiveinsulating layer and is developed thereon by finely dividedelectroscopic developing materials. The developed image may then befixed in place or transferred to a copy sheet where it is permanentlyfixed. Generally the photoconductive insulating layer is first chargedto sensitize it and is then exposed to a light image or other pattern ofactivated electromagnetic radiation to dissipate the charge in radiationstruck areas. Thus the charge pattern formed conforms to theelectromagnetic radiation pattern which impinges upon the plate. Thischarge pattern may then as above discussed be developed or made visibleby a chargewise deposition on the plate of an electroscopic orelectrostatically attractable, finely divided colored material which isreferred to in the art as toner.

As disclosed in the above noted Carlson patent, suitable inorganic andorganic materials may be used to form the photoconductive insulatinglayer on which the latent electrostatic image is formed. Otherphotoconductive materials have been disclosed in the prior art as beinguseful in similar electrophotographic processes such as in U.S. Pats.2,357,809; 2,891,001; and 3,079,342. Some of these materials arevitreous selenium, polymers such as polyvinylcarbazole, and resinsuspensions of inorganic photoconductive pigments such as, for example,zinc oxide and cadmium sulfide. While most of these materials haveevidenced some commercial utility, there are certain inherentdisadvantages to the commercial use of each of the suggestedcompositions.

The discovery of the photoconductive insulating properties of highlypurified vitreous selenium has resulted in this material becoming thestandard in commercial xerography. Vitreous selenium, however, issensitive only to wavelengths shorter than about 5,800 A.U. In addition,xerographic plates made with selenium are expensive to manufacture sincethis material must be applied to the supporting substrate by vacuumevaporation under carefully controlled conditions. Also, vitreousselenium layers are only metastable and may be re-crystallized intoinoperative crystalline forms at temperatures only slightly in excess ofthose prevailing in conventional xerographic copying machines.

Other known xerographic plates made with certain aromatic organicphotoconductors have relatively low sensitivity to light and have mostof this sensitivity in the ultraviolet range, which is not fullysatisfactory for use in conventional electrophotographic copying devices. Even the most sensitive organic photoconductive polymers leave muchto be desired for commercial purposes. The choice of materials availablefor use in aromatic polymeric plates is, of course, limited because ofthe necessity of the selection of an already photoconductive material.In addition, all of the above noted xerographic plates lack abrasionresistance and stability of operation particularly at elevatedtemperatures.

Binder plates containing zinc oxide pigments, while comparativelyinexpensive, are lower in sensitivity as compared with vitreous seleniumplates and are not reusable. Also, as above noted, their visiblesensitivity is quite limited. Furthermore, it is necessary to use suchhigh percentages of photoconductive pigment in order to attain adequatesensitivity that it is difficult in zinc oxide plates to obtain smoothsurfaces which lend themselves to efficient toner transfer andsubsequent cleaning prior to reuse. An additional drawback in the use ofcommercial zinc oxide binder type plates is that they can be morereadily sensitized by negative corona than by positive which results inpoor print quality. This property makes them commercially undesirablesince negative corona discharge generates much more ozone than positivecorona discharge and is generally harder to control.

In copending application, Ser. No. 518,450, filed in the US. PatentOfiice, Ian. 3, 1966, electrophotographic plates and processes employingphthalocyanine pigment dispersed in a binder material are disclosed.These plates may be reusable or non-reusable and have sensitivitieswhich extend over the entire spectrum. In copending application Ser. No.867,864, filed Oct. 16, 1969, abandoned for continuation-in-partapplication Ser. No. 889,718, filed Dec. 31, 1969, now Pat. No.3,640,710, commonly assigned, preferred phthalocyanine binderformulations are disclosed and claimed.

SUMMARY OF THE INVENTION Generally, phthalocyanine pigment-binder platesare prepared by incorporating phthalocyanine pigments in a dissolved ormelted binder through ball milling and/or pebble milling. These millingprocedures usually must be carried on for days in order to produceacceptable dispersions. Further, when ball milling and/or pebble millingis employed, recrystallization of the alpha-form phthalocyanine startingmaterial to the much more photosensitive beta polymorph is eitherextremely slow or does not take place at all.

It is, therefore, an object of this invention to provide a process forthe preparation of phthalocyanine pigmentbinder dispersions that isdevoid of the above disadvan tages.

Another object of this invention is to provide a much more rapid processfor the preparation of phthalocyanine pigment-binder dispersions.

Still a further object of this invention is to provide a rapid processof preparing photosensitive beta-forn1 phthalocyanine pigment-binderdispersions using the less sensitive alpha polymorph as a startingmaterial.

Yet another object of this invention is to provide a rapid and reliableprocess for the preparation of phthalocyanine pigment-binder dispersionswherein said phthalocyanine is present over a wide range ofconcentration.

A further object of this invention is to more economically produce anelectrophotographic plate comprising phthalocyanine pigment dispersed ina binder material.

The above objects and others are accomplished in accordance with thepresent invention, generally speaking, by providing a process for thepreparation of an electro photographic plate which comprises combiningphthalocyanine pigment particles and a binder material in a liquidmedium and sandmilling the combination.

This process is much more rapid than pebble milling and/or ball milling.In fact, dispersions are prepared hours rather than days. Moreover,through the use of sandmilling, as opposed to ball milling and/orpebblemilling, a less sensitive alpha phthalocyanine starting materialis recrystallized to the much more sensitive beta form in a surprisinglyshort period of time. In some instances sandmilling has been found to bethe only method that results in this conversion of alpha to betaphthalocyanine. Commercially acceptable dispersions may have a pigmentcontent of up to about 15-20% by weight. Whatever pigment concentrationis employed, however, for a given pigment concentration, dispersionsprepared by short term ball milling and/or pebble milling do not yieldelectrophotographically acceptable coatings. The above described processworks particularly well when one wishes to prepare a beta metal freephthalocyanine-binder dispersion starting with the alpha form to beconverted and amounts of beta, X-form or mixtures thereof. In additionto the alpha form which is to be con- 'verted, any suitablephthalocyanine may be employed. Typical phthalocyanines include metalphthalocyanines and metal-free phthalocyanines such-as alpha, beta andX-form. phthalocyanine.

Any suitable resin may be employed in the process of the presentinvention. Typical resins include petroleum hydrocarbons,styrene-acrylonitriles, epoxys, polycarbonates, polysulfones,styrene-butadiene copolymers, polyesters, phenolics, alkyds,silicone-alkyds, coumarone-indenes, phenoxys, polyvinylcar'bazoles andpolyurethanes. A preferred composition for use in the process of thepresent invention comprises a combination of a phthalocyanine pigmentwith an alkyd-acrylate resin blend, a silicone resin, and a chlorinatedhydrocarbon, more fully described in US. Pat. No. 3,640,710.

The sandmilling may be carried out for any suitable time. A preferredtime period ranges from about 0.2 hour to about 2.0 hours. Optimumresults are achieved when said sandmilling is carried out for about 0.75hour mploying about 50%.by volume sand and maintaining temperatures ofabout 120-180 F.

After the materials are combined to form a photoconductive layer, saidlayer may be positioned upon any suitable support substrate. Typicalsupport substrates inelude paper, aluminum, brass and plastics.

The pigment-binder-solvent slurry may be applied to substrates by any ofthe welleknown painting or coating methods, including spray, flowcoating, knife-coating, electro-coating, Mayer bar drawdown, dipcoating, reverse roll coating, etc. Spraying in an electric field may bepreferred for smoothest finish and dip coating for convenience in thelaboratory.

DESCRIPTION OF PREFERRED EMBODIMENTS To further define the specifics ofthe present invention the following examples are intended to illustrateand not limit the particulars of the present invention. Parts andpercentages are by weight unless otherwise indicated.

EXAMPLEI The slurry is milled for 1 hour at 2400 r.p.m. The tem- Ialpha-phthalocyanine is being finely and uniformly dispersed, it is'alsobeing consistently crystallized to the more photosensitive beta-formunder these conditions. The pigment dispersion is diluted to about 35%weight percent solids with toluene and then applied by Mayer Rod to aconductive substrate of about 5 mil aluminum foil. The resultantpigment-binder layer is bluish-green in color. The substrate is coatedto a dry thickness of about 0.3 mil. The photodischarge characteristicsof the pigment-binder coating is determined by corona charging the layerto about 500 volts positive (measured by a Keithley Model 610 BRelectrometer with DC probe) followed by exposure to a tungsten lamp(quartz iodine at 2850" color temperature). Under these conditions, anexposure of about 4 foot-candle seconds is suflicient to reduce thepotential to about 60 volts. Using conventional xerographic equipmentsuch as the Xerox Model D copier, the phthalocyanine pigment-bindercoating is similarly charged, exposed to a standard positive test targetwithtungsten light, and then developed with dry toner by conventionalcascade means. The toned image is then transferred to ordinary bondpaper. A high quality image is obtained.

. EXAMPLE 11 D equipment.

EXAMPLE III About 50 grams of the formulation described in Example I isplaced in a glass jar and milled for about 1 hour on a Gardner paintshaker using steel burnishing balls as a milling aid. The deep bluedispersion is then coated and tested as described in Example I. Thecoating accepts only about volts and discharges to about 60 volts withabout 2 foot-candle seconds of exposure. No image is obtained using theModel D equipment. Examples IV, and V illustrate cases where sandmilling a phthalo binder slurry gives an acceptable photosensitivedispersion where pebble milling does not. It is evident that for thesame pthalocyanine used, pebble milling is diflicult if not incapable ofconversion.

EXAMPLE IV ple I, the bluish-green layer is found to accept 500 voltsand discharge to 60 volts with 4.8 foot-candle seconds of exposure. Anelectrostatic image is produced, developed and transferred to paper asdescribed in Example I.

EXAMPLE V The following materials are combined in a gallon ball milljar, full of V2" diameter flint pebbles and are roller milled for about20 hours at about 140 r.p.m.:

144 g. of alpha-form metal free-phthalocyanine (obtained from AmericanCyanamid);

960 g. of Arotap EP-891 1-7-7;

328 g. of Silicone Resin SR-82;

144 g. of Syloid #244, a silica pigment (obtained from W. R. Grace andCo.);

960 g. of Chlorowax 70-LP; and

3000 g. of toluene.

The dispersion is coated as described in Example I. The coating is deepblue in color which is characteristic of the alpha form ofphthalocyanine. The material is found to accept only 240 volts andrequires about 3.6 foot-candle seconds of exposure to discharge to about60 volts residual. A suitable electrostatic image is not obtained on theModel D equipment. It is evident that only a portion of the pigment isconverted to the beta-form, thus limiting the photosensitivity of thecoating.

Although the present examples Were specific in terms of conditions andmaterials used, any of the above listed typical materials may besubstituted when suitable in the above examples with similar results.While sand is the preferred milling media other small bead-type mediamay be employed in the process of the present invention such as glassbeads, Coors Ceramedia and Minimedia (the latter two are small particleceramic beads). In addition other materials may be incorporated in thesystem of the present invention which will enhance, synergize orotherwise desirably affect the properties of the systems for theirpresent use. For example, a silica pigment may be incorporated in thesand milling process to serve as an antiblocking agent.

What is claimed is:

1. A process for the preparation of a photoconductive layer of anelectrophotographic plate which comprises combining a phthalocyaninepigment and a binder in a liquid medium, said phthalocyanine beingselected from at least one member of the group consisting of alpha,alpha and beta, and alpha and X-form phthalocyanine and sand milling thecombination until at least a portion of said alpha phthalocyanine isconverted to beta phthalocyanine.

2. The process as defined in claim 1 wherein said binder materialcomprises an alkyd-acrylate resin blend, a silicone resin, and achlorinated hydrocarbon.

3. The process as defined in claim 1 wherein said sand milling iscarried out for about 0.2 hour to about 2.0 hours.

4. The process as defined in claim 1 wherein said conversion is carriedout at a temperature of about 120 l80 and about 50% by volume sand isused in said sand milling operation.

5. The process as defined in claim 1 wherein said layer is deposited ona conductive supporting substrate material.

6. A process for the preparation of a photoconductive layer of anelectrophotographic plate which comprises combining phthalocyaninepigment particles said particles being selected from at least one memberof the group consisting of alpha phthalocyanine, alpha and betaphthalocyanine, and alpha and X-form phthalocyanine, an alkydacrylateresin blend, a silicone resin, and a chlorinated hydrocarbon and sandmilling the combination for about 0.75 hour in a sand mill filled byvolume with sand at a temperature of about -180 F.

7. The process as defined in claim 6 wherein said layer is deposited ona conductive supporting substrate.

8. The process as defined in claim 7 wherein said substrate materialcomprises a conductive paper.

9. The process as defined in claim 7 wherein said substrate materialcomprises a conductive paper.

10. A process for the preparation of a photoconductive layer of aneletcrophotographic plate which comprises combining phthalocyaninepigment particles said particles being selected from at least one memberof the group consisting of alpha, alpha and beta, and alpha and X-formphthalocyanine and milling the combination in a mill containing millingmedia said media being selected from the group consisting of glass beadsand small particle ceramic beads until at least portion of said alphaphthalocyanine is converted to beta phthalocyanine.

References Cited UNITED STATES PATENTS 3,296,008 1/1967 Pugin 260--314.53,357,989 12/ 1967 Byrne et al 260314.5

OTHER REFERENCES Chemical Eng. News, Nov. 25, 1957, p. 61.

WILLIAM L. JARVIS, Primary Examiner U.S. Cl. X.R.

